Transport of printable substrates formed by suction belts is known from the prior art, the function of which is to hold and keep a substrate against said belts as they move. This type of solution, well known to those skilled in the art, has a few disadvantages however. In particular, this type of device, causing movements of air and pressure gradients, can cause deformation of substrates if the latter are large in size. The accuracy of the printing will therefore be affected. On the other hand, the use of these suction belts with some wide used printing technologies, especially inkjet printing, can cause accidental aspiration of ink present in the printing heads and cause the deactivation of the printing heads. In the best of cases this type of incident needs reactivating of the heads, and in the worst of cases replacement of the deactivated heads, the latter now unusable.
To overcome these problems, especially to allow moving of substrates precisely, there are in the prior art techniques using cylinders comprising a plurality of clamps gripping the substrates by the front edge relative to the direction of moving of the substrate. Adapted to printing machines of inkjet type, this type of solution has several disadvantages however. Indeed, this system needs all the heads inkjet to be arranged orbitally around a large-sized cylinder. Moreover, this type of system poses the problem of the difficulty in adjusting the position of the printing heads. In fact, for quality printing, ink ejected from the printing heads must form a jet whereof the direction is perpendicular to the surface of the substrate. It is understood in this case that the use of a cylinder to transport and hold the substrate, whereof the surface by definition is not flat, means intricate adjusting of the position of the printing heads. For this same reason, it is difficult to use substrates of variable thickness, as the change of substrate means adjusting all the printing heads. On the other hand, the printing pitch, that is, the position of the clamps on the cylinder, is fixed, meaning that the printing rate remains the same regardless of the size of the substrate.
Also known from the prior art are substrate transport systems using chains or conveyors on which clamps are arranged to grip the substrates and transport them on a transport path whereof a portion is flat, resolving the problem of arrangement of the printing heads. However, this type of solution always has the problem of fixed printing pitch, which imposes a fixed rate and in this case poses the extra problem of not being able to use substrates of different sizes without stopping printing and proceeding with intricate adjusting of the position of the clamps.
More recently, the Applicant has proposed in its patent application WO2013156540 a device and method for transporting printable substrates precisely, adapted to substrates of various types, sizes and thicknesses, and to make prints at variable pitch. There is described a substrate transport system in a printing machine, along a transport path oriented according to a longitudinal axis from at least one entry store supplying the printable substrates to at least one exit store receiving the substrates, characterized in that it comprises mobile gripping means, each comprising an opening/closing system ensuring release or gripping of a substrate, said gripping means comprising front and rear gripping means, each gripping a part of the substrate, respectively front and rear of the same substrate, along the transport path, guide means for guiding the gripping means along the transport path, at least one motorisation means ensuring moving of the gripping means along guide means, preferably with independent moving between at least the front gripping means and the rear gripping means, the substrate transport system being adapted to grip each substrate so as to tense and/or move substrates, even if they have variable lengths, along the transport path, the guide means, the gripping means and their associated opening/closing system being controlled by computer facilities.
Although the device and method of WO2013156540 already meet a highly satisfactory manner the needs of the man skilled in the art, the Applicant has now developed a device and method that improve the transport of printable substrates on all workstations included in a printing machine, in particular a printing machine without contact with the substrate, such as inkjet printing machines.
Patent application US2015/0049150 discloses an apparatus for conveying a substrate, wherein the apparatus comprises rails and mobile units moving and maintaining the substrate that are configured to interact so that the moving of the mobile units is effected by magnetic interaction between the rail and said mobile units. The use of a low pressure rest (70), sometimes also referred to as a vacuum rest, is disclosed in the embodiments described in FIGS. 5 and 6.